1. Field of the Invention
The present invention relates to an actuator for a power antenna device of a motor vehicle, such as an automobile.
2. Description of the Related Art.
Power antennas which automatically extend and contract by drive motors are installed in many automobiles such as passenger cars. This kind of device requires speed reducer means to reduce the rotational speed. Referring to FIGS. 5 and 6, speed reducer means in a related actuator comprises: first reduction gears wherein the torque is transmitted from a worm 13 provided on the motor shaft to a worm wheel 14; and second speed reduction gears wherein the torque is transmitted from a pinion gear 15 through an idle gear 16 to a drive gear 17.
The related speed reducer means results in a rather complicated structure and requires many elements due to the three stages of gear engagement as described above. Also, the related speed reducer means is not suited for a resin casing which has lately been tested to obtain a lighter body. Ordinary resins are difficult to shape with high precision though the mounting precision of the gears is largely dependent on the molding precision of the resins. Ordinary resins have, as their properties, low strength and a tendency to plastically deform. Consequently, when gears are mounted on resin casing, the portions supporting the axles of the gears are deformed by the radial reaction force of torque transmitted by gear engagement. As a result, the interval of the axles will become wider and smooth gear-engagement will be affected. This phenomena becomes worse on a gear engagement at a larger stage in transmission. Special kinds of resin, such as engineering plastic, will supply the necessary accuracy and strength, but these special kinds of resins require special molding processes and thus increase the cost substantially.
The speed reducer means in the related actuator takes up a large space since the three stages of gear engagement result in a complicated structure with many elements that have to be arranged. A part of the speed reducer means inevitably is placed outside the space occupied by the cord case, which makes the casing large. For a lighter and compact body, this problem has to be solved.